Abstract

The Diarrhetic Shellfish Toxins (DST), okadaic acid (OA) + dinophysistoxin-1(DTX-1), were detected above the regulatory limit of 0.20 μg/g of digestive gland(DG) in (non-commercial) blue mussels (Mytilus edulis) from Sullivans Cove,Tasmania. Pectenotoxin-2 (PTX-2), PTX-2 seco acids and 7-epi-PTX-2 SA were alsodetected in mussels. This was associated with the occurrence of the toxicdinoflagellates, Dinophysis acuminata and D. fortii, which were seasonally prevalentat high cell densities (up to 7,380 cells/L for D. acuminata, 500 cells/L for D. fortii).A high density of D. truncata (1,850 cells/L) did not result in increased DST levels inM. edulis at Parsons Bay, Tasmania, suggesting that this may be a non- or weaklytoxic dinoflagellate.Subtle variations among Dinophysis morphotypes can pose problems for rapid andaccurate identification. Tasmanian sequences of the D1-D3 region of the large subunitrDNA of D. fortii were indistinguishable from those of D. fortii from France and D.acuta from the North Atlantic, while Tasmanian D. acuminata was indistinguishablefrom European and New Zealand D. acuminata. Genetic sequencing of New ZealandD. acuta failed to discriminate between Tasmanian D. fortii and New Zealand D.acuta and neither did sequencing discriminate between European D. fortii and D.acuta.A field depuration experiment was conducted in the Derwent River by placing M.edulis in 38 μm mesh size cages to screen out Dinophysis plankton cells. Musselsdisplayed biphasic depuration kinetics with a faster rate of PTX loss over the first 30days followed by an increase of OA + DTX-1 depuration once there was no furtherchange in PTX levels. The slow rate of depuration of OA + DTX-1 from day 15 to 30followed by an increase in depuration may be attributed to mussels using lipid storageduring a period of reduced food availability leading to a release of toxins in boundfractions. Solid Phase Adsorption Toxin Tracking (SPATT) detected dissolved DSTin the Derwent River seawater medium at levels as high as 0.34 OA + DTX-1μg/SPATT bag. Cellular and exuded toxicity of Prorocentrum lima varied between two culture strainsisolated from different locations in Tasmania, Australia. Cellular OA was greater inthe Little Swanport (PLLSP) strain (36 pg/cell) compared to the Louisville Point(PLLV) strain (3.8 pg/cell), which was the only strain producing DTX-1. PTX-2 wasproduced by both strains at small concentrations up to 1.2 pg/cell. This is the firstreported occurrence of PTX-2 produced by P. lima. The Louisville strain excretedhigher concentrations of OA (reaching 18 μg/SPATT bag) in the first 20 dayscompared to the Little Swanport strain (11 μg OA/SPATT bag). For both strains thisdeclined to 4 μg/SPATT bag on day 40. Both strains exuded higher dissolved toxinlevels at low cell abundance of 1,200 cells/L (PLLV strain reaching 1.6 μg OA +DTX-1/SPATT bag) compared to at 2,400 cells/L (0.4 μg OA + DTX-1/SPATT bag).Tasmanian strains of P. lima were more toxic than other global strains and poses apotential DSP risk to Tasmanian shellfish farms.In-vitro experiments with Prorocentrum lima suggest that dissolved toxins are exudedfrom DST producing dinoflagellates as well as from depurating mussels. Most of theDST was present dissolved in the seawater (94 %) when SPATT bags were exposedto P. lima cultures (6 % of DST in cells). Only a small amount of DST (1 %) wasdetected in the seawater medium when SPATT bags were exposed to contaminatedmussels (99 % of DST in mussels). OA displayed an increase by more than 0.11 μg/gDG in mussels immersed in dissolved DST for 48 hrs indicating that mussels canaccumulate DST in in-vitro conditions.Dissolved DST can pose an additional threat to shellfish farms and can extend harvestclosure periods after toxic dinoflagellate blooms. Toxicity differences amongdinoflagellate species and strains can pose problems for shellfish monitoringprograms and may require phytoplankton regulatory limits to be varied according tolocality.